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ENGINEERING AND FAMINE RELIEF             267

Hangchow, started in October, three months after his arrival, and
finished in April of the following year.

The problem was that these canals needed the sea water to carry
&e traffic, while the water itself brought in the silt. After careful study
Su Tungpp established the fact that it was important to keep the Salt
Canal, which ran through the busy sections of the city, free of muddy
water, while ways could be devised for the sea water to come in through
the other canal, Maoshan, which ran through the less populated district
on the eastern suburb. By building locks on the Chientang River in
the south, he could shut off the flow during high tide and release it
again at low tide. The two canals were connected in the north of the
city, and by the time the bay water had come through the canal in the
suburb, it would have travelled three or four miles and the sediment
already would have had a chance to settle. The Salt Canal, which had
to be kept clear, was four feet below the level of the other, so that
while the water from the suburban canal would partly supply the canal
in the city, it would give it water almost clear of silt. In order to
maintain the proper water level in the city canal, he also constructed
a canal outside the Yuhang Gate in the north of the city to join it
up with the lake. Thus a continual supply of water was assured, and
the cost and nuisance of dredging the Salt Canal inside the city was
avoided.

' The system worked, and he succeeded in giving the canals a depth
of eight feet, which, according to the elders of the town, had never
been achieved before.

Equally as important as the canal traffic itself was the problem of
water supply. Various attempts had been made to guide the fresh
water of West Lake, which came from the mountain springs, to the
city. Distributed in various sections of the city were six reservoirs* but
the fresh-water mains had frequently broken down. Eighteen years
earlier, when Su was deputy magistrate at this place, he had assisted
in the repair of the mains, but now West Lake had become shallow
through the ever-spreading growth of a kind of water plant whose
abundant roots, tangled in the mud, continually raised the lake bed.
The mains had broken down and the city people were drinking
slightly salty water, or had to buy water from the lake, costing a penny
a gallon. Su Tungpo consulted the surviving monk, now over seventy
years old, who had supervised the repair of the mains before. The
mains, made of sections of large bamboo pipes, could not last long.
Su Tungpo had the entire system replaced with strong clay pipes, pro-
tected on the top and bottom by flag-stones. It was an expensive pro-
ject, requiring the building of new sections sometimes three hundred
yards long, leading from one reservoir to another. Furthermore, he
guided the lake water to two new reservoirs in the northern suburb